This application pertains to the art of thermal cutoffs and, more particularly, to thermal cutoffs for protecting electric circuits. The invention is particularly applicable for use with thermal cutoffs of the type having a meltable thermal pellet, and will be described with specific reference thereto. However, it will be appreciated that the invention has broader aspects and can be used with other types of thermal cutoffs.
Application of a heat source to the outside of a thermal cutoff has long been recognized as a means of producing a time delay or a current sensitive fuse in conjunction with a thermally sensitive fuse. This has been done previously by wrapping a thermal cutoff body in dielectric tape, and placing a free standing resistance coil of fine wire over the tape. The realistic limit of resistance for this type of assembly is five ohms, because of the fragile nature of the fine wire coil. It would be desirable to provide a thermal cutoff with an external resistance heater having a substantially greater resistance than is possible with a fine wire coil.